To harness solar energy, it is necessary to collect heat in a system or convert it directly to electricity through silicon solar cells. Systems for collecting solar energy vary. One of the more conventional systems involves circulating a fluid, such as water, through a collector unit. In such a unit, the solar energy impinging on the collector unit, through a heat exchange process, is transferred by conduction to the fluid. Obviously, the heated fluid can then be circulated to put the retained heat to useful purposes.
Such collector units can be constructed of glass, metal or plastic. Glass collectors are prohibitively expensive. Due to costs and other factors, plastic collector units appear to have an edge on units constructed of metal. With proper construction and compounding, plastic collector units have the advantage of a much lower cost coupled with resistance to corrosion, scaling and mild freezing conditions. These latter factors are extremely important in a direct circulation system where the pool or tap water passes through the collector unit, rather than accomplishing a heat transfer in a secondary heat exchanger device remote from the collectors.
Plastics which have been found especially suitable for collector units include the polyolefins, and particularly, the low-cost polyethylenes and polypropylenes. These latter plastics can be compounded with stabilizers to reduce ultra-violet degradation and are serviceable in environments where there is direct sunlight impingement on their surfaces, even when glazed enclosures are used to achieve a hot-house effect about such collector units. Temperatures of the collector units can be over 200.degree. F. (93.degree. C.) in such systems.
With the proper design parameters, these polyolefins make very acceptable solar collector units; however, they have a drawback when collectors fabricated from them are to be joined with the more conventional plastic piping such as ABS and/or PVC. ABS plastics, a family of amorphous thermal plastics, are a combination of the monomers of acrylonitrite, butadiene and styrene which can be solvent-welded like PVC (polyvinyl chloride) plastics. However, neither PVC nor ABS piping and/or fittings can be solvent-welded to the polyolefins. As a result, some mechanical connection must be effected between a collector unit constructed of a polyolefin and a conventional ABS or PVC plumbing system.
Because the polyolefins mentioned above are soft, especially at elevated temperatures found in collector unit applications, conventional mechanical coupling devices are typically unacceptable. Fittings connected with screws in the outlet piping of such a collector unit typically separate due to the deformation of the polyolefin occurring where the screw loading is concentrated. If the service pressure of the fluid circulating in the collector unit is increased to normal household line pressure (approximately 80 psi), the problem becomes more acute. This cold flow tendency of plastics under load is greatly increased under the elevated temperatures experienced in solar collector unit applications.
As a result, this invention is directed to an improved internal mechanical connector for solar collector units fabricated from polyolefins or the like. In addition, using the new mechanical connector of this invention, the collector unit can be disconnected from the plumbing without any damage to it, which is not possible if solvent welding is employed.